Polymer beads with substantially uniform sized diameters, between 0.5-10 μm, are finding an ever-increasing number of applications in coatings, electronics, microelectronics, biomedical, and information technology. Particle size control and narrow size distribution are key parameters for most of these applications. Several routes have been used to synthesize mono-disperse polymeric particles. One method is seeded suspension polymerization which uses uniform particles as seeds that are swollen with monomers prior to conducting polymerization. The other method is dispersion polymerization, which is generally recognized as a type of precipitation polymerization conducted in the presence of a suitable polymeric stabilizer that is soluble in the reaction medium. Under favorable circumstances dispersion polymerization, in a batch step process, results in the preparation of polymeric particles, often mono-disperse particles, of 0.1-15 μm in diameter.
Dispersion polymerization was initially developed employing a hydrocarbon medium in the 1970's [see, Barrett, K. E. J.; Thomas, H. R. J. Polym. Sci., Polym. Chem. Ed. 1969, 7, 2621]; however it was an extension of the procedure to encompass polar solvents, such as ethanol or methanol, that greatly expanded the utility of this polymerization procedure [see, Tseng, C. M.; Lu, Y. Y.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Part A: Polym. Chem. 1986, 24, 2995]. El-Asser later extended the procedure to living anionic dispersion polymerization in hydrocarbon solvents [see, El-Aasser, M. S.; et. al. J. Polym. Sci., Part A: Polym. Chem., 1996; 34, 2633].
A dispersion polymerization is defined as a type of precipitation polymerization in which the monomer and all other reactants (including polymeric stabilizers) are initially soluble in the reaction medium, but the polymer is insoluble or substantially insoluble. Therefore a dispersion polymerization starts as a homogeneous solution polymerization but as polymer (or oligomer) chains grow in size they eventually reach a molecular weight higher than a certain critical value and precipitate from solution and aggregate to form colloidally unstable precursor particles. These particles coalesce and adsorb stabilizers from the reaction medium onto their surface until they become a colloidally stable dispersion of micelles in the reaction medium. At this point, the total number of particles in the system is fixed, and the nucleation stage ceases. Subsequent polymerization, also termed the particle growth stage, occurs predominantly inside the swollen nuclei or micelles but also in the reaction medium. However, the newly-formed polymers should not form additional nuclei but should be captured by existing particles [see, Kawaguchi, S.; Ito, K.; Adv. Polym. Sci., 2005, 175, 299].
The reaction is easy to carry out, lends itself to scale up and yields particles with a very narrow and uniform particle size.